Plant diseases which are caused by various pathogens such as, for example viruses, bacteria, oomycetes and fungi, can lead to, considerable yield losses in crop plant cultivation, which firstly has economic consequences, but, secondly, also endangers the safety of human nutrition. Since the last century, chemical fungicides have been employed for controlling fungal diseases. While the use of these substances has succeeded in reducing the extent of plant diseases, it cannot be ruled out even now that these compounds have a harmful effect on humans, animals and the environment. If the consumption of conventional plant protection agents is to be reduced to a minimum in the long term, it is therefore important to study the natural pathogen defense of various plants against different causative organisms, and to exploit them in a targeted manner for the generation of pathogen-resistant plants by means of recombinant manipulation, for example by the introduction of external resistance genes or by the manipulation of the endogenous gene expression in the plants.
There are only few approaches, which confer a resistance to pathogens, mainly fungal and oomycete pathogens, to plants. This shortcoming can partly be attributed to the complexity of the biological systems in question. Another fact which stands in the way of obtaining resistances to pathogens is that little is known about the interactions between pathogen and plant. The large number of different pathogens, the infection mechanisms developed by these organisms and the defense mechanisms developed by the plant families, genera and species interact with one another in many different ways.
The infections caused by the fungal, oomycete and bacterial pathogens lead to the activation of the plant's defense mechanisms in the infected plants. Thus, the activation of plant and animal innate immunity systems involves a specific detection of microbe-associated molecular-patterns (MAMPs) by different sets of host pattern-recognition receptors (PRRs). Several members of the plant leucine-reach-repeat (LRR) receptor-like kinase (RLK) family have been found to function as PRRs. Thus, FLS2 and EFR RLKs are PRRs for the bacterial MAMPs flagellin and EF-Tu, respectively, whereas LysM/CERK have been suggested to be the receptor for the fungal MAMP, chitosan. The initiation of the immune responses mediated by these PRRs involves the formation of MAMPs-induced complex with additional RLKs, the endocytosis of RLKs proteins and the activation of MAPK-signaling cascades. Some LRR-RLKs, such as FERONIA, BAK1 and ER; have dual functions controlling plant immunity and different cell growth and developmental processes. The molecular and genetic bases of this double functionality remain elusive.
Thus the ER protein negatively regulates, through its genetic interaction with two closely related paralogs (ERL1 and ERL2) and the Too Many Mouths (TMM) LRR receptor-like protein, several developmental processes such as stomatal patterning, inflorescence architecture, lateral organ shape, ovule development and transpiration efficiency. A MAPK signaling cascade, which includes the YODA MAP3K, and the MKK4/5 and MPK3/6 kinase modules, has been placed downstream of the receptors in stomata pattering, but biochemical and genetic interactions among these components have not been proved due to lethality-associated phenotypes of some of corresponding mutants.
Zygote development also depends on the YODA (MAP kinase signalling pathway). Loss of the MAPKK kinase gene YODA or the two MAP kinase genes MPK3/MPK6 blocks zygote elongation, such that the first division results in an abnormally small basal cell that typically fails to form a recognizable suspensor (Lukowitz, W., et al., Cell 116: 109-19 (2004)). Hyperactive forms of YODA have the opposite effect, causing abnormally long suspensors and often completely inhibiting growth of the proembryo (Lukowitz, W., et al., Cell 116: 109-19 (2004)).
Until now, the strategy for generating fungus-resistant plants has frequently involved the crossing-in of quantitative resistance traits (resistance QTLs). However, the disadvantage of this procedure is that undesirable traits are frequently also crossed in. Moreover, the breeding methods required are very complicated and time-consuming.